1. Field of the Invention
The present invention relates to an imaging optical system, and more particularly to an imaging optical system suitably applied to a solid-state imaging element such as a CCD image sensor or a CMOS image sensor. The present invention further relates to an imaging device incorporated with the imaging optical system, and a digital apparatus loaded with the imaging device.
2. Description of the Background Art
In recent years, as high performance and miniaturization of an imaging element i.e. a solid-state imaging element such as a CCD (Charged Coupled Device) image sensor or a CMOS (Complementary Metal Oxide Semiconductor) image sensor have developed, digital apparatuses such as mobile phones or personal digital assistants incorporated with an imaging device using such an imaging element have been widely spread. There is also an increasing demand for miniaturization and high performance of an imaging optical system (imaging lens) for forming an optical image of an object on a light receiving surface of the solid-state imaging element to be loaded in such an imaging device.
Japanese Unexamined Patent Publication No. 2010-026387 (patent literature 1) and Japanese Unexamined Patent Publication No. 2011-090122 (patent literature 2) disclose some of the examples of the imaging optical system. The imaging lens disclosed in patent literature 1 is provided with, in the order from an object side, a first lens element having a positive refractive power, an aperture stop for adjusting the light amount, a second lens element having a negative refractive power and concave toward the object side, a third lens element having a positive refractive power and concave toward the object side, and a fourth lens element having a negative refractive power. The imaging lens disclosed in patent literature 2 is constituted of four lens elements, i.e. is constituted of, in the order from the object side, a first lens element having a biconvex shape and having a positive refractive power, a second lens element having a biconcave shape and having a negative refractive power, a third lens element having a meniscus shape concave toward the object side and having a positive refractive power, and a fourth lens element having a negative refractive power. Further, an aperture stop is disposed at a position closest to the object side.
In the imaging lens disclosed in patent literature 1, the first lens element has a large curvature portion (i.e. a portion with a small curvature radius) facing the object side. Accordingly, in the case where the user intends to obtain a wide angle of view with use of the imaging lens, it is difficult to correct comma aberration or chromatic aberration. It is also difficult to shorten the total length of the imaging lens.
Further, in the imaging lens disclosed in patent literature 2, the refractive power (optical power) of the first lens element is extremely strong. This may make error sensitivity extremely high, and may increase the performance variation at the time of assembling into an imaging lens.